Process for lifting skin imprints



Unite This invention relates to a new process for lifting latent skin imprints from surfaces on which they are found.

It is frequently necessary to identify latent skin prints, present on furniture, walls, utensils, paper and other surfaces, for criminal and other detection purposes. Such prints are usually developed or made more clearly visible by dusting with a finely-divided powder which clings to the oils and other body exudates impressed in a characteristic pattern onto the surface by contact with the skin of the fingers, palm or foot.

Identification of the developed prints generally requires comparison with prints recorded in official files. To make this posa'sible, it is usually necessary to make an accurate duplicate of the dusted print on an easily carried, fileable surface. This has hitherto been done by photographing the dusted print or by lifting it from its surface site by applying to it an adhesive tape which, upon withdrawal, removes the dust from the surface in the characteristic pattern of the skin print. Photography fails where the surface is inaccessible to the camera or is so curved as to produce distortion in the picture. Lifting by adhesive tape can be employed in such situations but has certain undesirable features. Pressure must be applied to the tape to ensure its adherence to the surface and the dust. Such application of pressure tends to spread the dust which, while clinging to the latent print, is not tenaciously held to the surface. To lift the print in its original definition, it is also necessary to apply pressure on the adhesive tape uniformly over the entire area. This is often diflicult to do particularly if the surface is curved. As a result the dust pattern picked up on the tape is usually difiused and fuzzy. The prepared tape is generally obtainable only in sheets or strips of limited width so that where a large surface area is to be covered, as, for example, in the case of a large hand print, the tape must be applied in sections with resulting distortion. Although no difficulty is ordinarily experienced with cylindrical curved surfaces, some distorting wrinkling occurs when the flat, non-elastic tape is applied to substantially spherical surfaces. The sticky tape, furthermore cannot be easily handled or filed so that it must either be covered with a non-tacky transparent liner or photographed.

The object of this invention is to provide a method for lifting dusted skin prints from surfaces .of substantially any type, shape or size, while retaining the original clarity and definition of the dust-developed print.

Another object is to provide a means for lifting dusted latent prints which, per se, forms a clearly readable, permanent, readily fileable record.

ate t 2,986,831 Patented June 6, 1961 faces. It is, furthermore, easy to store, carry and use,

being appliable from a container by any suitable means as, for example, by spraying or by dropping a small amount on the surface and allowing the liquid to spread into a film.

The finely divided powder used for dusting the latent skin print can be any material conventionally used for the purpose, such as powdered graphite, charcoal, lampblack, aluminum or other powdered metal, boron nitride,

metal oxides such as magnesium oxide, flour, talc, and the like, which is not soluble in the subsequently applied liquid coating. The powder can be chosen to provide a contrasting color with the film. The powder is dusted on the surface bearing the latent skin print and the nonclinging excess removed in conventional fashion to develop the print, namely a print showing the characteristic pattern of the skin visibly outlined by the dust.

The liquid coating composition essentially comprises a natural or synthetic polymer settable into a solid, flexible, strippable film which is also preferably non-tacky. The hardened film must not adhere so tenaciously to the surface to which it is applied that it cannot be readily re moved or stripped. Flexibility is important so that it can be removed and manipulated without breaking or tearing. Substantial freedom from tackiness is also desirable since this improves its handling and filing characteristics. Although desirable, it is not essential that the set film itself be completely non-tacky, since tackiness of the formed film, in many cases can be eliminated by light dusting Still another object is to provide a process for lifting dusted skin prints which is simple, easy to apply by anystrippable, flexible, preferably non-tacky, film, and then stripping the hardened film from the surface. The liquid coating flows over and conforms to the contours of the dusted print pattern without disturbing it. When the coating sets into a solid film, the dust, in the characteristic pattern of the print, is firmly embedded therein,

with a finely-divided powder, such as talc or a conditioning agent which does not obscure the embedded print pattern. Polymers and copolymers having the desired characteristics per se or by modification with such components as plasticizers to improve flexibility and strength, tackiness inhibitors such as finely divided inorganic mate rials which are insoluble in the fluid composition, e.g. magnesium oxide and other metal oxides, talc, silica and carbon black, and the like, are well known to those skilled in the art.

"The liquid coating compositions can also advantageously be colored by incorporation of a suitable dye or finely divided pigment so as to provide a'marked contrast with the dust employed to develop the latent print, thereby increasing clarity and readability of the embedded print.

. The liquid coatingis preferably a fluid solution of a solid polymer dissolved in a volatile solvent, in which case setting of the applied coating can be rapidly achieved by evaporation of the solvent upon exposure of the large film surface to air. Such polymer solutions have the practical advantages of being preparable in the final form of use long in advance, of being packageable in a single container, and of requiring no further treatment to produce hardening. Films can be made from polymer solutions containing volatile solvents, which harden within a matter of minutes, in some cases as little as a minute or two. If desired, of course,-evaporation of the solvent and consequent hardening of the film can be speeded up by warming, but ordinarily this is not necessary.

Any natural or synthetic solid polymer can be employed forour purpose, which can be dissolved in a volatile solvent to form a fluid solution and which, after evaporation .of the solvent forms a strippable, flexible, preferably'non-tacky film, either iperse or by modification with conditioningv components, such. .as plasticizers dispersed tackiness inhibitors, and the like.

Illustrative examples of solid polymers, which can be employed in our process as solutions in:yolatile solvents,

includethe polyamideswhich are soluble in volatile solvents at ordinary temperatures, such as theN-alkoxyalkyl polyamide derivatives; acrylic and methacrylicresins and their, copolymers, such as polyacrylamide, polymerized unsaturated hydrocarbon and halogenated hydrocarbon;

propanol, isopropanol, acetone, methylene chloride, chloroform, trichloroethylene, carbon tetrachloride,- hexane, benzene, petroleum ether and other volatile petroleum solvents, methyl ethyl ketone, cyclohexane, tetrahydrofuran, etc. In some cases, cosolvent systems, such asv methylene chloride-methanol, chloroform methanol, ethanol-water, can be used for such reasons as improved solubility and reduced viscosity.-

The concentration of the dissolved polymer is not critical and is dictated only by such practical considerations asthe thickness of-film desired, the optium setting time for particular applications and the viscosity of the solution; About 5 to solutions areusually ade-:

quate. In general, it is not desirable to increasethe concentration of the polymer'to the point'where viscosityis so excessive as to make application ofa thin coating difficult. The concentration at which this occurs varies, of-eourse, with different polymers and different solvents.- Pl'as ticizers suitable for the particular polymer can be dissolved in the polymer-volatile solvent-solution. Some examples of a few of the plasticizers which "can be employed include dibutyl and dioctylphthalate,.dibutyl and diethyl sebacate, trioctyl, triphenyland tricresylphosphate, butyl stearate, diacetin, high molecular weight alcohols, such as octanol, decanol, and the like.-

Other conditioners, such as tackiness inhibitors, can also be dispersed in the polymer solution. In general, this will not be necessary,since-suitable polymers which form non-tacky films can readily be selected.-

Polymerizable liquids which can be. cured into a solid, strippable, flexible film after application to a surface can 4 a o a. p r ly redli uid n ym rts sh.ass ttaian ta. tially polymerized polyamidesya catalyst introduced into a liquid monomer, a mixture of copolymerizable monomers, or a partially cured liquid polymer such as a liquid siloxane, liquid polysulfide polymer, liquid polyester, liquid epoxy, and liquid polyamide; or a reactive atmospheric component such as oxygen or moisture, as in the case of partially cured polyurethanes which harden by reaction with moisture in the air, Such polymerizable liquids are entirely feasible for our purpose, although, in some instances they may require longer setting times than volatile solvent solutions of solid polymers or mixing with a copolymerizablecomponent or a catalyst shortly prior to use.

The liquid coatings can be applied to the surface bearing the dusted skin prints in any suitable and convenientmanner, although care should be taken not to apply it, in so forceful a manner as to disturb the dusted printas, V for example, by hard brushing. The liquid can be flowed. on as, for example, by dropping small amounts on the sur face and permitting it to flow into a film.

The most convenient mode of application is by spray-- ing, which can be accomplished in any desired manner. The solution can be sprayed from a manually operated container or from one pressurized by an inert gas such as nitrogen or carbon dioxide. It can also, in some cases, v be sprayed as an aerosol, Where the particular polymer solution is miscible with a propellant agent maintained in the liquid state at normal temperatures by the application of pressure. Such propellant agents include, for example, methyl'chloride and various chlorofluoro methanes and ethanes, such as dichlorodifiuoromethane (Freon-12), monochlorodifluoromethane (Freon-22), and dichlorotetrafiuoroethane (Freon-114) EXAMPLE I N-methoxymethyl polyarnide solutions were prepared bydissolving 10 grams. of the polyamide in 100 ml. of methylene chloride/methapolin ratios varying from 40:60- to 135, These formed clear, non-gelling solutions of low viscosity, which couldbe sprayed onto a surface to form thin, flexible, non-tacky, strippable solid films inabout 2 to 10 minutes. When the solutions were applied to a surface carrying fingerprints dusted with finely divided powders, such as carbon black or talc, the dust patterns embedded in the stripped film duplicated perfectly the pattern originally developed on the surface. The stripped films, when mounted on cards, formed permanent, fileable records of the lifted prints.

Table I summarizes other film-forming polymer solutions which we have prepared and found suitable for our also be'employed. The curing agent can be beat applied purpose. Table l Polymer Concen- Solvent Other additives tratlon a Polyisobutylene 2 Do 10 1% o Polyisobutylene 2 I Mlerocrystalllne wax 10 phr.

l l chloride 5 Eth i chi ide/ at he 1 e n ene or ac o O 5 'o Dlbutyl sebacate? l0 phr.

20 Methylenechloride 23 .do 20 I Di-2ethylhexylphthalate 5phr. 20 Di-2-ethylhexyl phthalate 10 phr. 20 Di-Z-ethylhexyl phthalate i 15 phr.. 20 20 Di-Q-ethylhcxyl phthalate 5 phr. 20 Di-2-ethylhexyl phthalate 10 phr. 20 Dl-Zethylhexyl phthalate 15 phr. 10 do 10 Cyggghexanone/methyl chloride 10 Tetrah ydrofuran .4. 10 Methylene chloride.

S'ee footnotes at end of table.

Table I-Continued Polymer Concen- Solvent Other additives tration Methylene chloride ..do 30 40 10 15 20 Do 30 Chlorosulfonated polyethylene (Hypalon). 10 2-chl0ro-l,3-butadiene polymer (neoprene) 10 do Balata 10 Carbon tetrachloride Grams per 100 ml. solvent.

1 Polymers having molecular weights of 100,000, 120,000, 150,000, 200,000, and 300,000 were all employed in preparing the filmiorming solutlo 2 Molecular weight 100,000. 3 Parts per 100 parts resin. Vinyl chloride/vinyl acetate 90/10 5 Fluorinated vinyl chloride-vinyl ester copolyrner.

Tacklness inhibitor. 1 Plastieizer to improve flexibility.

EXAMPLE II The following commercially obtainable polymer solutions can be sprayed to form non-tacky, strippable, flexible films, which are excellent for our purpose:

Cocoon 560 and Cocoon 501, solutions of vinyl chlo' ride-vinyl acetate copolymer in methyl ethyl ketone.

S-llS-E, a solution of a terpolymer of vinyl chloride, vinylidene chloride and methyl acrylate.

Hysol 6101-B, a solution of a partially cured epoxy resin which drys to form a non-tacky film.

EXAMPLE III A solution of N-methoxymethyl polyamide was prepared by dissolving 10 grams of the polymer in 100 ml. methanol/methylene chloride, 60/40. The solution was loaded into an aerosol container and pressurized with dichlorodifluoromethane (Freonl2) in a ratio of 50 parts polymer solution to 40 parts propellant liquid. The aerosol mixture was readily sprayed, forming films from 2 to 4 mls. in thickness with two passes of spray. The films were tack-free in less than 2 minutes and could be stripped in less than 10 minutes. The lifted imprints were clear and sharp.

EXAMPLE IV A commercially available aerosol solution of a polyacrylamide (Krylon #1301) produced satisfactory films when given a drying time of one-half hour after each pass.

EXAMPLE V Peroxide polymerization catalysts were added to the following liquid silicone polymers and the catalyzed liquid applied as a film and allowed to set.

(A) Silastic QS5330. Curing was complete in 5 minutes.

(B) Silicone 81712. Curing was complete in 1 hour.

The resulting films were flexible, non-tacky and strippable.

Although this invention has been described with reference to illustrative embodiments thereof, it will be apparent to those skilled in the art that the principles of this invention can be embodied in other inventions but within the scope of the claims.

We claim:

1. A process for lifting a latent skin imprint from a non-skin surface which comprises developing said imprint by dusting with a finely divided powder, coating the surface carrying the dusted imprint with a liquid composition consisting essentially of a polymer settable into a solid flexible, substantially non-tacky, strippable film, said dusting powder being insoluble in said liquid composition, setting the coating into a solid film, and separating the" film from said surface, said stripped film carrying embedded therein the finely divided powder in the pattern of said skin imprint.

2. A process for lifting a latent skin imprint from a non-skin surface which comprises developing said imprint by dusting with a finely divided powder, coating the surface carrying the dusted imprint with a liquid composition consisting essentially of a solution of a solid polymer dissolved in a volatile solvent, said liquid coating, upon evaporation of the volatile solvent, forming a solid, flexible, substantially non-tacky, strippable film, said dusting powder being insoluble in said liquid composition, permitting said coating to set into a solid film, and separating the film from said surface, said stripped film carrying embedded therein the finely divided powder in the pattern of said skin imprint.

3. The process of claim 2 in which the liquid coating composition is a solution of a polyamide which is soluble in a volatile solvent at ordinary temperatures.

4. The process of claim 3 in which the liquid coating composition is a solution of an N-alkoxyalkyl polyamide in a mixture of methylene chloride and methanol.

5. The process of claim 4 in which the N-alkoxyalkyl substituent is N-methoxymethyl and the ratio of methylene chloride to methanol is from about 40:60 to :35.

6. The process of claim 5 in which the polymer solution is admixed with a miscible propellant liquid which is maintained in the liquid state at normal temperatures by the application of pressure.

7. The process of claim 6 in which the propellant liquid is dichloro-difluoromethane.

8. The process of claim 2 in which the polymer is a. vinyl polymer.

9. The process of claim 8 in which the polymer is a vinyl chloride copolymerized with vinyl acetate.

10. The process of claim 2 in which the polymer is a vinyl polymer containing chlorine and fluorine substitucuts.

11. The process of claim 1 in which the polymer is a liquid silicone.

References Cited in the file of this patent UNITED STATES PATENTS 1,259,981 Hedrick Mar. 19, 1918 1,497,972 Bock June 17, 1924 1,833,287 Hadley Nov. 24, 1931 1,937,575 Joyce Dec. 5, 1933 1,951,203 Pitman Mar. 13, 1934 2,020,376 Rich Nov. 12, 1935 2,238,073 Patten Apr. 15, 1941 (Other references on following page) 7 3,. E UNITED STATES PATENTS FOREIGNPATENTS 2,312,913- Kirby -Mar. 2, 1943v 594,075 Grgaglirjtain Nov. 3, 1947 2,313,807: Gurry Mar. 16, 1943 r 2,615,820 Schwoegler Oct. 28, 1952 OTHER REFERENCES 2,634,216 Pineles er a1. Apr. 7, 1953 Surface Finish of Reduction Gear Teeth, by J. A. 2,639,216 Barth May 19, 1953 Davies, dated 1945. Class 18-'--Reshape Digest. 2,744,628 Engstrum Dec. 18, 1 956 2,819,196 Munro Jah. 7, 1958 

1. A PROCESS FOR LIFTING A LATENT SKIN IMPRINT FROM A NON-SKIN SURFACE WHICH COMPRISES DEVELOPING SAID IMPRINT BY DUSTING WITH A FINELY DIVIDED POWDER, COATING THE SURFACE CARRYING THE DUSTED IMPRINT WITH A LIQUID COMPOSITION CONSISTING ESSENTIALLY OF A POLYMER SETTABLE INTO A SOLID FLEXIBLE, SUBSTANTIALLY NON-TACKY, STRIPPABLE FILM, SAID DUSTING POWDER BEING INSOLUBLE IN SAID LIQUID COMPOSITION, SETTING THE COATING INTO A SOLID FILM, AND SEPARATING THE FILM FROM SAID SURFACE, SAID STRIPPED FILM CARRYING EMBEDDED THEREIN THE FINELY DIVIDED POWDER IN THE PATTERN OF SAID SKIN IMPRINT. 